1. Field of the Invention
The present invention relates to compositions for enhancing the durability of the adhesive bond between silicone elastomers and substrates. More particularly, it relates to improved primer systems for silicone elastomers used as interlayer materials in the lamination of plastic or glass substrates.
2. Description of Related Art
Thermoset polydimethylsiloxane-based materials are widely used as elastomers, adhesives, sealants, potting compounds, and interlayers. These siloxane materials are normally formulated from one or two component liquid precursors that can be processed by casting, molding, calendaring, extrusion, or other methods known in the art. Depending upon the degree of cross-linking and reinforcement, these cured polysiloxanes can range from very soft materials to relatively hard elastomers.
Transparent silicone materials based on thermoset polydimethylsiloxane copolymers containing silicon-bonded vinyl and hydride functional groups, and which are generally cured by a hydrosilation reaction mechanism catalyzed by a platinum compound, are often used as interlayers to join transparent glass or plastic sheets in laminated transparencies for aircraft and other applications. Reinforcing materials may be added to these elastomers to enhance their strength and toughness.
Transparent silicone interlayers of this type can be placed between a pair of glass or plastic substrates, whether flat or shaped, and joined by conventional lamination procedures. The silicone interlayers may be either pre-formed sheet silicone or formed by casting, i.e., cured-in-place, using liquid silicone precursors. Because adhesion between the cured interlayer and the substrate tends to be very poor unless an effective primer is used, a primer is applied to each substrate surface before the silicone interlayer is laminated between them.
Known primers used in the preparation of transparent laminates containing silicone interlayers, such as AP-132 and AP-133, organosilane primers sold by Lord Chemical, SS-4120 primer sold by General Electric, and 1204 and 1205 primers sold by Dow Corning, provide good initial adhesion, but do not sustain adequate bond durability for extended use outdoors. With these conventional primers, the adhesive bond between the silicone and the substrate is degraded by prolonged exposure to moisture. This often results in delamination. The deleterious effect of water exposure is especially pronounced with glass substrates as well as with glass or plastic substrates having thin film coatings consisting of metals or metal oxides, or both.
Accordingly, the need exists for improved environmental durability of the adhesive bonds between silicone elastomers and glass or plastic substrates.